Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
The choice of a colorant in ink jet systems is critical to image quality. For colors such as cyan, magenta, yellow, green, orange, etc., the peak wavelength (.lambda.-max), the width of the absorption curve and the absence of secondary absorptions are important. The colorant should also have a high degree of light fastness after printing onto the ink-receiving element. For aqueous dye-based inks, the dye needs to be sufficiently soluble in water to prepare a solution that is capable of producing adequate density on the receiving element and stable for extended periods of storage without precipitation. High quality ink jet printing with dye-based inks requires dyes which will provide both bright hue and good light stability. It is difficult to find dyes, particularly yellow dyes, which meet all of these requirements.
Aqueous dye-based inks for high-quality, photo-realistic, ink jet printing require water-soluble dyes with excellent color and high light- and water-fastness. Typically the dyes are chosen from acid, direct and reactive dyestuffs developed for the dyeing of natural fibers such as paper, wool and cotton. Water solubility of these dyes is due to the incorporation of negatively charged substituent groups such as sulfo or carboxy.
U.S. Pat. Nos. 5,851,274 and 5,810,916 disclose inks containing water soluble dyes including Acid Yellow 23 (tartrazine) and Acid Yellow 17. While these dyes have excellent yellow color and water solubility, there is a problem with these dyes in that they have poor light stability.
U.S. Pat. Nos. 3,891,619; 4,000,123; 4,954,563 and 4,997,921 disclose water-soluble azoindole dyes useful for dyeing polyamide fibers. Similar dyes are described in DE 3,643,462 for use in conventional printing inks. Water-insoluble phenylazoindole dyes are disclosed in JP10-251,577 for use in ink jet inks. However, there is a problem with these dyes in that they are limited for use in organic solvent-based inks or in aqueous pigmented inks and there is no disclosure that they would be suitable for aqueous dye-based inks.
It is an object of this invention to provide bright, readily water-soluble and light stable azoindole dyes useful for aqueous ink jet printing.